This invention relates to a magnetic resonance imaging apparatus and method using a nuclear magnetic resonance (hereinafter referred to as "NMR") phenomenon to obtain an NMR spectrum or tomographic image of a desired inner region of an object under examination non-invasively. In particular, it relates to a method for obtaining a substantially high uniformity of a static magnetic field in the apparatus.
The magnetic resonance imaging (hereinafter referred to as "MRI") apparatus uses an NMR phenomenon to obtain an NMR spectrum or tomographic image of a desired inner region of an object under examination non-invasively.
An object to be examined is placed in a uniform static magnetic field generated by a permanent magnet or a superconductive magnet. A high frequency magnetic field with a frequency equal to that of the Larmor precession (Larmor frequency .omega.) of a specified atom, e.g., hydrogen (proton), constituting tissues of the object (e.g., human body) is applied to excite the nuclear spins and NMR signals emitted from the excited spins are measured as free induced decay or spin echo signals when the spins return to the ground state. Here, the Larmor frequency .omega. is calculated by the following formula and depends on the intensity of the magnetic field. EQU .omega.=.gamma..multidot.Ho
In the above equation, .gamma. is a magnetic rotational ratio inherent in each atomic nuclei. Ho is the intensity of the static magnetic field.
In ordinary MRI, gradient magnetic fields are superimposed on the static magnetic field to produce spatial difference in the magnetic field and a measuring sequence is repeated at a predetermined repetition time TR while adding location information to the NMR signals, thereby obtaining a plurality of NMR signals required for an image of one slice or spectrum. The static magnetic field in such MRI is required to be highly uniform in order to obtain high spatial resolution with a small gradient magnetic field.
Recently, fast imaging techniques have been developed and applied to clinical applications including a technique in which a plurality of echo signals with different phase encodes are measured within a single repetition time (JPA No.60-42906) or echo planer imaging (EPI) in which all of the phase encoded signals required for one image are measured at single excitation of the spins. These fast imaging techniques require still higher uniformity of the static magnetic field. As shown in FIG. 7, in the fast imaging technique in which the numbers (n) of echo signals are generated by switching the polarity of a readout gradient magnetic field and added with different phase encodes, the number n depends on decay characteristic of the signals. The phenomenon of an excited spin returning to its original state is called "relaxation" and when the nuclear spins are assumed to be macroscopic magnetization, relaxation in which the longitudinal magnetization component is in accord with the direction of the static magnetic field is called "longitudinal relaxation" and relaxation in which the lateral magnetization component is zero is called "lateral relaxation (T2 relaxation)." The relaxation rate of the T2 relaxation becomes rapid if the uniformity of the static magnetic field is bad. The relaxation occurring in a non-uniform static magnetic field is called "effective lateral relaxation (T2*)." When the effective lateral relaxation rate is rapid, the number n of echo signals measured at a single excitation is limited. Therefore, in order to measure more echo signals at a single excitation, the effective lateral relaxation rate should be slow, that is, the static magnetic field should have high uniformity.
Apart from this, a technique has been developed in which protons of adipose tissue are selectively saturated using the fact that resonance frequency .omega.' of protons of adipose tissue differs from that .omega. of protons of the other tissue (mainly water molecules). In this technique, since the difference between the resonance frequencies (.omega.'-.omega.) is approximately 4 ppm, the value of uniformity of the static magnetic field must be better than 4 ppm. However, it has been difficult to attain uniformity of this order for every examined object and, as a result, the signals from adipose tissue are not sufficiently suppressed in some cases.
To meet the requirement for the high uniformity of the static magnetic field, a conventional MRI apparatus is provided with shim coils for generating correcting magnetic field between the magnet for the static magnetic field and the coils for generating the gradient magnetic field. The shim coils consist of a combination of a plurality of coils such as X, Y. Z, X.sup.2, Y.sup.2. Z.sup.2 . . . , which correct local deviation of the static magnetic field. The uniformity of the static magnetic field, which is several ppm/40 cmdsv for a superconductive coil having high uniformity, is improved to some extent (around 1 ppm) by incorporating a plurality of shim coils.
A technique for separating protons of fatty acid group from those of hydroxy group of water using chemical shift (.delta.) information included in NMR signals (spectroscopic imaging) has been developed and utilized for diagnosis of specific diseases (see, for example, JP No7-51124, J. Japan Magnetic Resonance Medical Science "Quantitative measurement of adipose using chemical shift imaging--Study on Duchenne-type muscular dystrophy" issued 1991). Since the proton chemical shift usually ranges around 7 ppm (maximum 20 ppm), a uniformity of the static magnetic field of better than 7 ppm is required. However, as mentioned above, the conventional MRI apparatus with shim coils achieves uniformity of only several ppm and thus chemical shift information of higher resolution can not be utilized.
Further, when a human body is the object to be examined, the rate of magnetization varies from place to place depending on the difference of tissues, blood or gases in the coelome or the like. This variation in the rate of magnetization leads to local changes in the uniformity of the applied static magnetic field. This variation can not be corrected by the shim coils, because it occurs locally. Thus, use of high-resolution spectrum has been severely limited in medial diagnosis.
On the other hand, the NMR spectrometer utilized for measuring high resolution spectrum in the field of analytical chemistry is provided with shim coils of various shapes, which are applied with an electric current to generate a magnetic field for correcting the ununiformity of magnetic field of the space where the samples are placed. Uniformity of the magnetic field ranging 10.sup.-7 .about.10.sup.-8 can be obtained by the shim coils. In addition, the samples are subjected to high speed rotation so that all of the spins in the samples are subjected to an average of the uneven magnetic field intensity. This improves the substantial uniformity of the magnetic field in the samples to a level of 10.sup.-9 (0.001 ppm) and enables a high resolution spectrum to be obtained.
However, this method can not be applied to an MRI apparatus where the object to be examined is a human body, while rotation of the magnet or shim coils can not be realized because of their weight.